Such a process takes the form of dividing up the original image into image elements or "pixels", generating an array of image values representative of the content of these pixels, processing these pixel image values so as to remove or reduce unwanted components therein and constructing an array of pixel image values corresponding to the desired processed image.
Images, in particular those scanned and displayed electronically, may be degraded by several forms of noise, including:
(1) Broadband noise affecting all pixels, for example photographic grain. A variety of image processing techniques have been proposed using filtering and transform methods for the suppression of broadband noise, e.g. in U.S. Pat. No. 4,442,454
(2) Noise consisting of randomly distributed modifications of image values corresponding to single or small groups of pixels. Such an effect has been referred to as "salt and pepper" noise because it may consist of random white and/or black spots. These may arise at various stages, including:
(a) in the formation of the original image, PA1 (b) during image sampling, or PA1 (c) in image data handling.
Methods to detect and remove isolated spot defects are described in Rosenfeld, A. and Kak, A. C., "Digital Picture Processing", Academic Press, 1976, and Pratt, W. K., "Digital Image Processing", Wiley, 1978. These procedures calculate the difference between the central pixel value and either the mean or the extreme of its neighbourhood pixels. The central pixel is replaced by a local average value if the difference exceeds an absolute threshold.
Such a technique inevitably results in loss of overall sharpness of the image as a large amount of high frequency information is lost especially in cases where the nature of the image and the spot defects necessitate the use of a low threshold.